In many circuit arrangements, a logic signal must be transmitted between two circuits that must otherwise be electrically isolated from one another. For example, the transmitting circuit could utilize high internal voltages that would present a hazard to the receiving circuit or individuals in contact with that circuit. In the more general case, the isolating circuit must provide both voltage and noise isolation across an insulating barrier. Such isolation circuits are often referred to as “galvanic isolators”. One class of galvanic isolators is based on transforming the logic signal to a light signal that is then transmitted to an optical receiver in the receiving circuit that converts the optical signal back to an electrical signal. The transmitting and receiving circuits are typically on separate substrates and connected to separate power supplies. While a significant number of commercial galvanic isolators are based on such optical transmission, optical galvanic isolators have a number of problems. For example, these circuits are electrically inefficient. Only a fraction of the signal power can be converted to an optical signal using a light emitting diode or similar device. At the receiver, the fraction of the power in the light signal that is recovered by the photodetector is very small. Hence, optical galvanic isolators require high power and signal amplification. In addition, the level of isolation provided by the circuits is limited by RF fields generated in the transmitting circuit and in the ambient environment, which are received by the circuitry in the receiving circuit and which are not related to the signals that are being sent optically. In principle, a conductive barrier can be utilized to shield the receiving circuit; however, such barriers block a portion of the light in the optical signal, and hence, further reduce the electrical efficiency of the isolator.
To overcome these limitations and others, a class of galvanic isolators based on one or more electrical transducers have been developed. One example is a transformer. In these galvanic isolators, the transmitter drives the primary winding of a transformer and the receiver is connected across the secondary winding. Typically, the transmitter and the two windings are constructed on a first semiconductor chip and the receiver is constructed on a separate chip that is connected to the first chip by wire bonds or the like. The two transformer windings are, typically, deposited over the drive circuits on the first chip by patterning two of the metal layers that are typically provided in conventional semiconductor fabrication processes.
The size of the transmitter chip is set by the size of the transformer coils, which typically require a significant area of silicon compared to the drive circuitry. The cost of the semiconductor substrate is a significant fraction of the cost of the isolator. This is a particularly significant problem in devices designed to operate at relatively low frequencies where large coils are required to provide the coupling between the transmitter and receiver. In addition, many applications require multiple independent galvanic isolators on a single substrate. Cross-talk between the isolators constructed on silicon substrates using conventional semiconductor fabrication techniques is difficult to block in a cost-effective manner because of fringe fields generated by one coil being coupled to an adjacent coil. If the chips are separated by a sufficient distance on the silicon substrate, the cost of the wasted silicon becomes significant.
In addition to the wasted silicon area, devices constructed using conventional silicon integrated circuit fabrication have limitations that are imposed by the design rules of the fabrication line and the limitations as to materials that are allowed on that line. For many applications, the dielectric insulation between the coils of the transformer must withstand voltages in excess of 1000 volts. The thickness of dielectric that is available in conventional CMOS fabrication lines is insufficient to provide this degree of insulation. In addition, in some applications, it would be advantageous to provide a ferrite layer between the coils of the transformer to improve the coupling efficiency. However, the materials in question cannot be utilized in many conventional fabrication lines.